Mannan-binding lectin (MBL) is a plasma collectin (protein with both collagen-like and C-type lectin domains) synthesised by hepatocytes and secreted into the blood stream. MBL is thought to have an important role in the innate immune system via the MBL pathway of complement activation. MBL is a multimeric molecule that can bind to a wide variety of bacteria and other microbes, neutralising them and/or opsonizing them by activating complement using the lectin pathway of complement activation. MBL binds to repeating mannose and N-acetylglucosamine sugar motifs characteristically displayed in high density on bacteria, fungi, viruses and protozoa but not on mammalian cells (Minchinton et al., 2002). It has been suggested that MBL plays an important role in the first hours/days of any primary immune response to a sugar-decorated pathogen. This provides the host with a first-line of defence before the adaptive immune system becomes operative. In addition, in humans MBL may be particularly important between 6 and 18 months of age when the adaptive system is still immature.
The human collectin genes are all located in a cluster on chromosome 10 (q21-24) (Hansen and Holmskov, 1998). There are two human MBL genes, MBL-1 a pseudogene and MBL-2 which encodes a protein product. MBL-2 comprises four exons with exon 1 (FIGURE) encoding a signal peptide, a cysteine-rich region and part of the glycine-rich collagenous region. Exon 2 encodes the remainder of the collagenous region and exon 3 encodes an a-helical coiled-coil structure that is known as the ‘neck’ region. The fourth exon encodes the carbohydrate-recognition domain that adopts a globular configuration.
MBL deficiency has been reported in many different populations and is largely explained by three structural and three promoter mutations. The structural mutations occur at high frequency (generally 15% or greater cumulative allele frequency in most population studies) and are single base changes in codons 52, 54 and 57 of exon 1 (nucleotides +154, +161 and +170 respectively). The changes are: Arg-52 to Cys (R52C, MBL D variant), Gly-54 to Asp (G54D, MBL B variant) and Gly-57 to Glu (G57E, MBL C variant). The A variant is wild-type MBL and O refers to all the variants combined (Madsen et al., 1994; 1995). The B and C variants have disrupted Gly-Xaa1-Xaa2 repeats of the collagenous region resulting in an altered capacity to form the collagen triple helix. The D variant introduces an additional cysteine residue and so may disrupt oligomer formation by generation of additional disulphide bands. The B variant mutation occurs in 22-28% of Eurasian populations, whereas the C variant mutation is characteristic of sub-Saharan African populations in whom it reaches frequencies of 50-60%. The D mutation reaches frequencies of 14% in European populations but can be much lower elsewhere.
In addition to the above structural gene mutations, several polymorphisms have been described in the promoter region and the 5′ UTR of the MBL gene (Madsen et al., 1995). These are the H/L, Y/X and P/Q loci at positions −550, −221 and +4 respectively of the MBL gene. The three loci are closely linked and four promoter haplotypes (LXP, LYP, LYQ and HYP) are commonly found.
Due to linkage disequilibrium, only seven haplotypes: HYPA, LYPA, LYQA, LXPA, LYPB, LYQC, and HYPD are commonly found (Minchinton et al, 2002). Two other very rare haplotypes have been described: HXPA in three African-American patients with systemic lupus erythematosus (Sullivan et al., 1996) and LYPD recently found in an Euro-Brazilian individual (Boldt and Petzl-Erler, 2002).
The concentration of MBL2 in serum is highly variable between healthy individuals. This variation is highly genetically determined by the presence of promoter, 5′ UTR and exon 1 polymorphisms. Previous studies have indicated that the three structural variants B, C, and D and some of the promoter haplotypes have a dominant effect on the MBL concentrations in serum. Genotypes made up of combinations of seven haplotypes are mainly responsible for a 1000-fold concentration variation found in healthy human beings. All three exon 1 variants are associated with significantly decreased MBL levels compared with homozygotes of the wild-type gene. For example, the relatively common A/B heterozygotes generally have around a tenth of the MBL concentration found in A/A individuals, while B/B homozygotes or compound variant heterozygotes (B/C, etc.) typically possess MBL levels around the limit of detection by enzyme linked immunosorbent assay (ELISA) methods. These variant structural alterations cause a disruption in initiating the collagen formation as. such, preventing proper trimer formation resulting in a non-functional MBL2 peptide. These structural abnormal peptide chains are shown to render MBL more susceptible to matrix metalloproteinase proteolysis resulting in diminished MBL2 measurable in serum. The dimorphic H/L and Y/X loci allow modulation at the transcriptional level, with associated production H>L and Y>X (Kilpatrick, 2002a). It has been well established that high MBL2 producing haplotypes are HYP, followed by LYQ and LYP, whereas the LXP haplotype is associated with the lowest level of serum MBL2. The LX promoter is shown to have an influence on the MBL2 level similar to that found in individuals with the B structural gene variant (Steffensen et al., 2000).
Several studies have shown that deficiency of soluble MBL2 in the bloodstream increases the overall susceptibility of an individual to infection and may constitute a significant risk factor when immunity is co-compromised. Several independent reports have shown that low MBL2 concentrations are involved in recurrent infections, especially in children and immunocompromised individuals such as cancer patients undergoing chemotherapy. Furthermore, MBL2 can affect the course of autoimmune diseases, cystic fibrosis, and is possibly implicated in recurrent miscarriage. The role of MBL2 in immunodeficiency virus (HIV) infection has attracted much attention and has resulted in somewhat conflicting findings. The role of MBL2 in relation to viral hepatitis is also under debate. In general, chronic infection with either hepatitis B or hepatitis C virus was generally associated with lowered MBL (Kilpatrick, 2002a).
In contrast, there is evidence that for some intracellular parasites such as Leishmania, MBL deficiency may be protective and this might explain the high frequency of MBL mutations in sub-Saharan Africa and South America.
Increasingly, there is evidence that the association between MBL levels and disease is complex. A number of publications have now appeared which suggest that MBL is also able to modulate disease severity in both infectious and autoimmune disease (Turner, 2003). The mechanism whereby MBL exerts such effects is unclear but one possibility is through a dose-dependent modulation of pro-inflammatory cytokines. Lanzrein et al. (1998) observed a lowered level of MBL2 in the CSF of Alzheimer's disease (AD) patients compared to control subjects. Since the serum levels of MBL2 were not changed in the same patients, this reduction of MBL2 in CSF appears to be linked to a higher degree of MBL consumption connected with complement activation in AD patients. Sofar, no genetic association studies with MBL have been reported in AD or other neurological diseases.
Alzheimer's disease (AD) is an age-related, progressive neurodegenerative disorder characterised by irreversible cognitive and physical deterioration. The incidence of AD increases with age, affecting 1 out of 10 persons older than age 65 and nearly 1 out of 2 persons older than age 85. Overall, the natural history of the disease can be characterised as an irreversibly progressive brain disorder that ultimately results in devastating memory loss, profound behavioural and personality changes, and severely damaged cognitive abilities. These impairments are related to the underlying death of brain cells and the breakdown of communication between them. In view of the large expenses for health care systems that must provide institutional and ancillary care for the AD patients, the impact of AD on society and on national economies is enormous.
Epidemiological studies have demonstrated several known or potential risk factors in AD, including advanced chronological age, female gender, low education level and positive family history of dementia (Alloul et al., 1998; Katzmann and Fox, 1999; Radebaugh et al., 1999; Launer et al., 1999). Of the different genetic markers identified, the most important risk factor to date is Apolipoprotein E (Apo E). The ε4 allele is recognised as a susceptibility gene for early-and late-onset familial AD as well as for sporadic AD (Corder et al., 1993; Tsai et al., 1994; Roses, 1996; Higgins et al., 1997). More recently, other polymorphisms in the promoter region of the Apo E gene have been found to be associated with AD (Mui et al., 1996; Artiga et al., 1998; Lambert et al., 1998a; b). In autosomal dominant early-onset AD, mutations in 3 additional genes have been identified, the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes (Campion et al., 1999; Finckh et al., 2000). The presenilin-1 (PSEN1) genotype and a CYP46 polymorphism have also been associated with a higher risk of late-onset sporadic AD (Wragg et al., 1996; Papassotiropoulos et al., 2003). The relative contribution of APP, PSEN and CYP46 mutations is, however, the subject of considerable controversy and the involvement of other genetic factors is suggested.
Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS). Predominantly, it is a disease of the white matter tissue. In people affected by MS, patches of damage called plaques or lesions appear in seemingly random areas of the CNS white matter. At the site of a lesion, a nerve insulating material, called myelin is lost (i.e. demyelination). People with MS can experience partial or complete loss of any function that is controlled by, or passes through, the brain or spinal cord.
The most obvious risk factor for MS is gender (female sex). In all studies, MS affects women more than men. Other risk factors include ethnicity (MS is most common in Caucasian people of northern European origin and extremely rare among Asians, Africans and Native Americans) and family history. People who have relatives with MS are more likely to develop the disease than people with no family history of MS. It seems clear from various population studies that there is a genetic susceptibility involved in contracting the disease.
Several other neurological disease have been associated with certain genetic risk factors. Dementia with Lewy bodies (DLB), for example, is an illness that presents with progressive dementia or psychosis. Parkinsonian signs, which may be absent or mild at the onset, eventually become common and rigidity is usually severe. Lewy bodies are found profusely in the brainstem, basal forebrain, hypothalamic nuclei and neocortex. Dementia with Lewy bodies is characterized by the relative absence of tangles and hyperphosphorylated tau in the brain. Parkinson's disease (PD) is a type of Lewy Body disease occurring in the middle or late life, with very gradual progression and a prolonged course. It can be considered as an example of neuronal system disease, involving mainly the nigrostriatal dopaminergic system. Elevated frequencies of the common CYP2D6 mutant allele, CYP2D6B, have been found among PD patients compared to controls, with an approximate doubling of risk for subjects homozygous or heterozygous for this allele (Armstrong et al., 1992; Stefanovic et al., 2000). Several other genetic markers have been variably associated with PD such as glucocerebrosidase ( Lwin et al., 2004), monoamine oxidase A (Hotamisligil et al., 1994; Nanko et al., 1996), monoamine oxidase B (Kurth et al., 1993; Costa et al., 1997), dopamine receptors and transporters (Nanko et al., 1994; Le Couteur et al., 1997; Plante-Bordeneuve et al., 1997), and catechol-O-methyl transferase (Hoda et al., 1996). Despite these different associations of genetic risk factors with certain neurological diseases, there is a continuous search toward more accurate genetic markers that provide a reliable prediction of the risk to develop a neurological disease.